Genetic and biochemical methods are being used to study the chemotactic behavior of Escherichia coli. The goal of this work is to understand the molecular basis of sensory perception and stimulus transduction in this system. Mutants defective in different aspects of the chemotactic response have been isolated and characterized and a tentative model of information flow through the chemotaxis machinery has been devised. The model distinguishes two components of the chemotactic response: a rapid excitatory phase in which chemical stimuli trigger changes in the pattern of flagellar rotation; and a slower adaptation phase which culminates in return of flagellar behavior to the pre-stimulus state. Mutants lacking only the adaptation phase of the response are altered in their ability to add or remove methyl groups on several membrane proteins required for excitation. The control of the methylation-demethylation reactions is being investigated by analyzing gene product interactions through reversion studies and by isolating deletion or insertion mutants totally devoid of enzymatic activity.